雖然地球上水資源豐沛，但由於全球地理環境之影響，造成水資源分配不均，以致局部地區與國家長年受缺水之苦，舉凡中東及北非地區是眾所皆知之事實。如何將取之不盡、用之不絕之海水轉換為淡水，且設計出熱交換器最佳操作性能，以降低投資與操作成本，產生最大經濟效益，是近幾十年來科學家研究之目標。本研究以低壓二效淡水機為主，探討各效之真空壓力對總造水量、總熱傳量及總熱傳面積之影響。其研究結果顯示升高各級熱流體進口溫度、熱容流量、採用較佳之總體熱傳係數、將第二效之真空壓力愈低、或提高第一效熱流體進口溫度，可得較佳之總造水量及總熱傳量，以致獲得最小之總熱傳面積。此外，真空壓力愈低，雖可獲得最小總熱傳面積，但是此亦會增加真空泵之消耗功率，在設計時必須同時考慮此項因素。

While abundant water resources on earth, the uneven distribution of water resources, which was caused by the effects of global geographical environment, results in localized regions and countries suffering from water shortages for years. It is well known that these regions cover the Middle East and North Africa. How to convert the inexhaustible seawater to fresh water and obtain the greatest economic benefits is thus become the goal of scientists for the recent decades. In this study, a low-pressure two-effect distillation plant is investigated theoretically. The effects of total amount of water, heat transfer rate, and total heat transfer area are taken into account. The results show that the optimum conditions are influenced by the overall heat transfer coefficient, the vacuum pressure of the second stage, and the inlet temperature of the heating medium in the first stage. In addition, the lower the vacuum pressure, the smaller the total heat transfer area. However, this may increase the power consumption of the vacuum pump.